No minister would hazard the revenge of a bloodstained leader unless he judged that the risks involved in staying at his post were even greater.

So Riyad Hijab will have deserted as prime minister of Syria because he thinks that President Bashar al-Assad is bound to be overthrown. After four years as a provincial governor and cabinet minister, he is better placed to make that judgement than any foreign diplomat or outside observer.

The real significance of Mr Hijab's defection is that it betrays the future expectations of a man who knows how Syrian politics work. Few believe that his departure will, of itself, trigger the fall of Mr Assad. What matters is that a man chosen to be prime minister clearly believes this outcome will happen.

And that leads to the next stage of reasoning. If Mr Hijab has concluded the ship is sinking, many of his former colleagues will be asking themselves whether they should also jump. Defections tend to come in batches: the resignation of Syria's ambassador to Baghdad last month triggered three more departures from the diplomatic corps. In the Syrian army, generals have defected in groups over the last 12 months.

So Syrian and foreign observers will be waiting to see whether Mr Hijab's decision causes other ministers to change sides. But that possibility will also be painfully apparent to Mr Assad. His security agencies can be relied on to redouble their efforts to prevent any further defections. Just as the prospect of ministers jumping ship becomes more likely, so the risks involved tend to become even more forbidding.

Barely two months ago, all Syrian ministers were summoned to hear their leader address them on the occasion of the appointment of a new government on 6 June. In their Damascus offices today, these apparatchiks will be acutely aware of the constant surveillance they must be under, with punishment awaiting any sign of disloyalty.

Perhaps the most devastating consequence of high level defections is the poisonous atmosphere of suspicion and mistrust they leave behind.

As for the practical value of Mr Hijab's departure, he will probably know little about the military response to the uprising. Any hopes that he will reveal the secrets of Mr Assad's security planning will be disappointed.

But he will know about the full impact of sanctions on Syria's economy and the condition of the government itself. As a bureaucrat rather than a politician, he will be familiar with the health, or otherwise, of the state machine. As such, Mr Hijab's defection amounts to another dent in the edifice of an otherwise implacable regime.